This invention relates to a rolling device and its components, and in particular relates to an improvement in material for the components of various kinds of rolling devices such as long-life and low-cost rolling bearings made of a bearing steel utilized for automobiles, construction equipment, machine tools, etc., and also rolling devices used as comparatively small-sized precision bearings such as spindle support bearings of a hard disk drive (HDD), a videocassette recorder (VCR), etc., and bearings of a cooling fan motor, etc.
Generally, as well as in the present invention, for example, rolling bearings, ball screws, linear guides, ball bushes, ball splines, etc., are well-known as representative or typical rolling devices. In addition, the following rolling members (of the rolling devices) are well-known.
Races (inner races, outer races) and rolling elements (balls, rolling bearing rolls) are known as components of the rolling bearings. Nuts, screw shaft, and rolling elements (balls) are known as components of the ball screws. Sliders (nuts), guide rails, and rolling elements (balls) are known as components of the linear guides. Bearings, guide races, and rolling elements (balls) are known as components of the ball bushes. Ball bushes, spline shafts, and rolling elements (balls) are known as components of the ball splines.
In the rolling devices, the rolling members are referred/defined as parts/components for making up each rolling device. The rolling member comprises the rolling elements and components rotated, slided or moved relative to the rolling elements for guiding the rolling elements. The rolling elements and the components are referred/defined as mated members relative to each other.
The races and rolling element of a bearing are utilized as components of a rolling bearing which is one example of the rolling devices. The races and rolling element make rolling motion between rolling surfaces and receive a contact stress repeatedly, and also they make sliding motion between a flange portion of each of the races and a rolling element of a roller bearing such as a cage or a component part of a rolling device. For the materials, hardness for resisting against a load, long rolling fatigue strength, good wear resistance to slide, and the like are required. Since the bearings are highly precise mechanical parts in their dimensions, sufficient dimensional stability is required. As the material of the members, generally SUJ2 is used for bearing steel or a steel equivalent to SCR420 is used for case hardening steel.
The loss and gain in using the materials of SUJ2 and SCR420 for rolling bearings are described below:
Since SUJ2 contains much C and Cr in alloy constitution, macro carbide or segregation easily occurs at the steel making time and soaking, etc., is performed to remove it, leading to an increase in the material cost as compared with SCR420.
Generally, inner and outer races for small bearings are molded by hot (warm) forging, then they undergo softening successively; because of the carbon amount of the material, SUJ2 has a high hardness after softening, is hard to cut, and increases the working cost as compared with SCR420. However, SUJ2 is only hardened and tempered to provide hardness required for a bearing; SCR420 is carburized or carbonitrided, then is hardened and tempered to provide hardness required for a surface layer, thus drastically increasing the heat treatment cost as compared with SUJ2. As a result, SUJ2 has a total bearing manufacturing cost lower than SCR420.
However, for the rolling bearing function, the bearing provided by carburizing or carbonitriding SCR420 contains much .gamma.R (retained austenite) on the surface layer and a residual compressive stress also occurs, thus the bearing made of SCR420 tends to have a long life particularly under lubrication mixed with foreign material and also show good fatigue strength and impact strength. SCR420 is superior to SUJ2 in dimensional stability of an important function as a bearing.
On the other hand, for the rolling elements, cold wire drawing material, which will be hereinafter called coil material, is used as a material and is molded like a rolling element by cold forging, which will be hereinafter called header working, and cutting is not performed. Comparing cold workability between SUJ2 and SCR420, SUJ2, which contains much C and Cr in alloy constitution, has a high hardness after annealing and is inferior to SCR420 in cold workability. However, SCR420 requires carburizing or carbonitriding, thus increasing the heat treatment cost as with the inner and outer races. As a result, SCR420 increases the manufacturing cost drastically as compared with SUJ2.
For the rolling element function, the rolling element provided by carburizing or carbonitriding SCR420 still contains much .gamma.R (retained austenite) on the surface layer and a residual compressive stress also occurs, thus the rolling element made of SCR420 tends to have a long life particularly under lubrication mixed with foreign material.
In contrast, rolling members of spindle support bearings of HDDs and VCRs, air conditioner fan motor bearings, etc., are used under a comparatively light load, but low vibration and sound produced by the bearing itself are required and in some cases, sound degradation during use may become higher than the upper limit of the usage. This means that a good sound characteristic rather than rolling fatigue strength is required for comparatively small precise ball bearings used for these applications.
Vibration and sound are produced by ball bearings for various reasons; they are produced by comparatively small precise ball bearings used for HDDs, etc., because vibration and sound occurring from the bearing raceway surface increases. That is, if the inner and outer race material of a bearing contains a large retained austenite amount, an impression is easily made on the raceway surface of the finished product of the bearing, thus load or impact load easily causes the bearing raceway surface to be deformed permanently. Such permanent deformation increases vibration and sound occurring from the bearing raceway surface, degrading the sound characteristic. Thus, reducing the retained austenite amount (.gamma.R) or setting the amount (.gamma.R) to zero is effective as a sound degradation countermeasure.
A bearing for which sound characteristic degradation is prevented by reducing or setting to zero the retained austenite amount in the material is disclosed in the Japanese Patent Unexamined Publication No. Hei. 7-103241.
As described above, for the conventional bearing member, case hardening steel which is carburized or carbonitrided has a higher bearing function than bearing steel, but rolling bearings made of bearing steel are often adopted giving cost priority under clean oil bath lubrication or grease lubrication.
However, the conventional rolling member made of bearing steel is high in material cost and also has poor cold workability as described above, and thus it is difficult to meet severe cost reduction demand in recent years. This is a first problem in the prior art.
Further, particularly, external water, etc., easily enters rolling bearings used with machines mainly run outdoors, such as bicycles, automobiles, agricultural machinery, and construction equipment, thus it is known that generally the life of the rolling bearings is shortened ("Jidousha no tripology" Shadan houjin jidousha gijyutukai, 1994. p.272). As its causes, it is known that the lubrication state becomes faulty or damage occurs due to corrosion such as a rust, etc., because water, etc., enters ("Korogari jikuuke handbook" J.Blendline et al., 1996, p.382). To prevent trouble caused by entry of external water, etc., sealing is enhanced for axle bearings and water pump bearings of engine auxiliary machinery for preventing water, etc., from entering.
However, if sealing is enhanced for improving hermeticity, air in the bearing expands and shrinks and water is sucked in because of the temperature difference during the operation and stop of the bearing, thus life lowering caused by water entry cannot perfectly be prevented. To cope with life lowering caused by water entry, high alloy steel such as stainless steel can be used as the bearing material to improve corrosion resistance, but the bearing cost increases. This is a second problem of the prior art.
Furthermore, it is also effective as sound degradation countermeasures to reduce or reduce to zero the retained austenite to cause permanent deformation to easily occur on the bearing raceway surface because of load or impact. However, the conventional reduction method is to perform subzero treatment or tempering treatment at high temperature, thus resulting in the cost equal to or greater than the current cost. This is a third problem in the prior art.
Countermeasures for reducing vibration or sound resulting from the bearing raceway surface are not originally taken; this means that the sound characteristic still has room for improvement.